Edge-emitting diode lasers generally produce laser beams with an elliptical cross-section and a large divergence in the growth direction, i.e., perpendicular to the substrate upon which the layers of the laser are grown. For example, semiconductor quantum well lasers with output wavelengths of about 870-980 nanometers (nm) generally have an optical field distribution spot size of approximately 0.4-0.6 micrometer (μm), while the size of the quantum well in the growth direction is only about 6-10 nm. Light with wavelengths of about 870-980 nm is thus strongly diffracted, producing a large beam divergence of approximately 30-40° in the growth or transverse (vertical) direction. This large beam divergence degrades the coupling efficiency of the laser diode with single mode optical fibers. There is therefore a need for efficient diode lasers with transverse beam divergences of at most about 20-30°.
The beam divergence of a diode laser can be reduced by spreading the optical field over a larger range of depths with respect to the substrate or growth direction. However, the optical field is restricted by waveguiding resulting from confinement layers located above and below the active layer containing the quantum wells from which photons are emitted. These confinement layers are generally about 1-1.5 μm thick and their refractive index is lower than the refractive index of the active layer. If the optical field distribution is spread, the thickness of these confinement layers usually needs to be increased correspondingly because the decay of the optical field distribution in these confinement layers should be large enough to prevent significant leakage into the high refractive index, high loss n++ GaAs substrate, p++ GaAs contact layer and metal layers. Performance parameters such as internal loss, efficiency and threshold current are degraded by optical leakage into such lossy layers. However, the thickness of the confinement layers is limited by the total thickness that can be practically grown. For example, extremely thick layers tend to develop undesirable morphological features such as pinholes, and undulations, and suffer from particulate contamination. Moreover, thick layers generally have unacceptably high series resistance.
It is desired, therefore, to provide a diode laser with a transverse beam divergence of less than about 28° that alleviates one or more difficulties of the prior art, or at least to provide a useful alternative to existing diode lasers.